Add roadmap for houdini

[gregwhitworth]

We need to define the pipeline for not only authors but discussions during the meeting. When will the apply hooks be called, for example.

Additionally, we should show dependencies from one piece to another to help prioritize.

[svgeesus]

@gregwhitworth This issue has, sadly, just sat here with no actual responses. I think that you are asking for better overall documentation on how to submit a proposal, is that correct? If not, could you clarify your request?